The inputs of digital circuits, for example the inputs of gate arrays, are often preceded by trigger circuits that have switching hysteresis, firstly in order to increase the signal-to-noise ratio, particularly in the vicinity of the switching threshold, and secondly in order to enable the processing of input signals with slower edges as well. Preferred trigger circuits with switching hysteresis, also known as Schmitt triggers, are known for instance from the publication entitled Halbleiter-Schaltungstechnik [Semiconductor Circuitry] by U. Tietze and Ch. Schenk, 5th Edition, 1980, page 135. A Schmitt trigger is an emitter-coupled pair of transistors which is essentially operated with a constant emitter current, which has one load resistor in each load circuit of the pair, and which has positive feed back provided by a voltage divider between one of the two load circuits and the corresponding input circuit. If the circuit is suitably dimensioned, then it is possible to provide for the constant emitter current to change from one transistor to the other upon toggling, without driving the pair of transistors to saturation. As a result, there are no storage effects upon the changeover, and substantially higher switching frequencies are attainable.
However, it is precisely with gate arrays, as with numerous other digital circuits, that the signal levels associated with the logic states within the circuits often do not match the signal level at the input, making a level conversion necessary. Moreover, adaptation of the hysteresis course to the behavior of the input signal is also necessary in many cases. However, in the known trigger circuit, the degree of saturation of the transistors, the output signal level and the course of the hysteresis cannot be adjusted independently of one another, so that adjustment of the trigger circuit becomes problematic.
It is accordingly an object of the invention to provide a trigger circuit with switching hysteresis, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and with which the degree of saturation of the transistors, the output signal level and the course of hysteresis can be adjusted independently of one another.